In the backlight of a liquid-crystal display device, increase in temperature of the device due to heat generation of a light source is monitored using a temperature sensor, and electric power according to the temperature is supplied. In relation to this, the following patent literature 1 proposes a technique in which electric power supplied to a cold-cathode tube is restricted when increase in temperature of the device is not detected while the electric power is supplied to the cold-cathode tube as a light source.